Ambulatory medical devices include implantable medical devices (IMDs) and wearable medical devices (WMDs). Some examples of IMDs include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of implantable medical devices include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
WMDs include wearable cardioverter defibrillators (WCDs) and wearable diagnostic devices. WCDs can be monitoring devices that include surface electrodes. The surface electrodes are arranged to provide one or both of monitoring to provide surface electrocardiograms (ECGs) and delivering cardioverter and defibrillator shock therapy.
Some medical devices detect events by monitoring electrical heart activity signals. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, medical devices can detect abnormally slow heart rate, or bradycardia. Some medical devices detect abnormally rapid heart rate, or tachyarrhythmia. Tachyarrhythmia includes ventricular tachycardia (VT) and supraventricular tachycardia (SVT). Tachyarrhythmia also includes rapid and irregular heart rate, or fibrillation, including ventricular fibrillation (VF).
Treatment of cardiac disease with medical devices may be complicated due to over-sensing by sense-amplifiers. Over-sensing occurs when sense-amplifiers incorrectly interpret sensed signals or noise as cardiac signal artifacts, such as P or R-waves indicative of atrial or ventricular heart contractions respectively. Over-sensing can result in a bradycardia device incorrectly inhibiting pacing therapy or a tachyarrhythmia device incorrectly delivering high energy shock therapy. The present inventors have recognized a need for improved sensing of events related to cardiac activity.
An example of a method of curvature based classification of cardiac signals can be found in Sweeney et al., “Curvature Based Methods for Selecting Features From Electrophysiologic Signals for Purpose of Identification and Classification,” U.S. Pat. No. 6,684,100, filed Oct. 31, 2000. An example of a method of determining whether noise is present on a cardiac signal based on a moving count of turning/inflection points can be found in Palreddy et al., “Cardiac Rhythm Management System With Noise Detector Utilizing a Hysteresis Providing Threshold,” U.S. Pat. No. 6,892,092, filed Oct. 29, 2001.